The invention relates to a shaft sealing ring of the kind defined in the preamble of claim 1.
In such a shaft sealing ring, known from DE 23 39 353 C3, the sealing lip is made of a material which is permeable to the medium to be sealed off, for example a fleece or nonwoven. A coating is applied to the surface of the sealing lip facing the sealed space so as to prevent leakage across the sealing lip.
The limits of operating parameters of a machine unit, such as gearing, converters, and the like often are set in consideration of shaft sealing rings. It is the chemical and thermal resistancey of the material of the sealing lip facing the medium to be sealed off which determines such limits. And it must be taken into account that, in the area of contact with the seal, the temperature of the medium to be sealed off rises still further due to frictional heat.
For example, according to German industrial standard DIN 3761 a mean temperature of 130xc2x0 C. in the oil sump should not be exceeded when sealing off hypoid oils by means of shaft sealing rings having sealing lips which are made of fluoroplastics (FPM) and are adapted to withstand maximum loading. In the case of sealing off ATF oil, however, the admissible mean temperature is 150xc2x0 C. Moreover, a rotational speed of 9000 r.p.m. should not be surpassed with typical shaft diameters for the use of radial shaft sealing rings in automatic transmissions. This corresponds to a circumferential speed of clearly less than 30 m/sec.
It is an object of the invention to devise a shaft sealing ring which provides reliable sealing and sufficiently long service life even when the temperatures and circumferential speeds mentioned above are exceeded.
This object is met by claim 1.
When using the shaft sealing ring in an automatic power transmission, for instance, the free surface of the sealing lip of the shaft sealing ring facing the sealed space is exposed to the medium to be sealed off. Medium to be sealed off penetrates the material of the sealing lip, as this is permeable to the medium to be sealed off, and thus reaches the sealing surface. The medium to be sealed off is conveyed along the sealing surface to be returned into the sealed space by the centrifugal forces acting by virtue of the rotation of the support body with the sealing member. In this manner the medium to be sealed off which is located in the contact zone of the sealing ring undergoes quick exchange and, therefore, heat dissipation is rapid.
The return flow of medium to be sealed off, as generated by the centrifugal force and caused by the permeability of the material of the sealing lip, making its way from the sealed space through the sealing lip to the sealing surface and back into the sealed space, assures continuous lubrication of the sealing surface with fresh medium to be sealed off. Hereby, friction is reduced at the sealing surface and the countersurface and, as a consequence, the frictional heat and the resulting rise in temperature of the medium to be sealed off are curbed.
As a result of the quick exchange, the medium to be sealed off is exposed for much shorter periods to the higher temperatures at the sealing surface and countersurface. The frictional heat generated at the sealing surface is dissipated continuously from the sealing surface zone by the quick exchange of the medium to be sealed off.
A shaft sealing ring according to the invention may be used also at high operating temperatures and elevated circumferential speeds without costly structural modifications. Satisfactory longevity of the shaft sealing ring can be warranted for rotational speeds as high as in the order of 12000 r.p.m. and for shaft diameters in the range of 60 mm.
A preferred material for the sealing lip is a nonwoven material which is compatible with the medium to be sealed off. Suitable materials are mechanically bonded nonwovens, preferably needled nonwovens. A mechanically bonded nonwoven containing polyester fibers is preferred. Nonwovens bonded by elastomers, especially by latex are suitable as well.
An essential difference of the shaft sealing ring according to the invention as against the one disclosed in DE 23 39 353 C3 resides in the fact that the sealing lip is not protected by a coating or cover impermeable to the medium to be sealed off. In spite of the fact that, in principle, medium to be sealed off thus may get by the sealing lip from the sealed side to the air side of the seal, leakage in operation nevertheless is prevented by the effect of the centrifugal force which diverts the flow of medium to be sealed off in radial outward direction.
Advantageous modifications of the invention are indicated in the dependent claims.